Lift-off deposition techniques are in general usage in the semi-conductor industry. Lift-off has, until very recently, only been possible with evaporation based deposition processes.
Lift-off refers to the process step in the production of semi-conductor devices which consist of removing a photo resist mask together with any surface material which has been deposited thereon, usually by some form of solvent method. During the fabrication process, as is well understood, the deposited material covers not only the area on a substrate where the material is desired but also actually covers the mask which must be removed together with the unwanted material before the next step in the deposition or manufacturing process can take place. Since the mask is covered with the deposition material such as various semi-conductor metal layers, insulators, etc., the solvent can only reach the mask at the edges of the mask structure where, theoretically, the mask is not coated with the deposition material. In order for this process to work it is necessary that the edges of the mask either be vertical or slightly overhang the unmasked areas to assure that an uncoated edge of the mask is available to and subject to the action of the solvent. It is accordingly very desirable, and in fact necessary, to only utilize those processes in the deposition procedures which will only coat the planar surfaces and not the edges of the masks.
Accordingly, lift-off has until very recently only been possible with evaporation-based deposition processes as mentioned previously. The evaporation deposition process approximates a point source operating in an environment of low pressure (&lt;10.sup.-3 Torr) resulting in a "line of sight" deposition. This is because the deposited films tend to coat only the planar surfaces of the substrate mask assembly leaving the edges uncoated. Stated very simply, this is a characteristic of the evaporation process.
It is well known that various types of sputtering processes produce much more rapid deposition rates which wold theoretically greatly reduce process times and thus production costs. However, attempts to use sputtering in the past have resulted in very poor lift-off.
With known sputtering processes such as magnetron sputtering, extremely dense clouds or fogs of the material to be deposited were formed in the reaction area due to the extremely high pressures necessary to achieve the sputtering reaction. The existence of this fog caused sides as well as planar surfaces to be plated thus resulting in the lift-off difficulties alluded to above. Recent developments have allowed the extension of lift-off techniques into the sputtering area by the development of hollow cathode enhanced processes are references [1,2,3] are set forth subsequently.
The hollow cathode enhanced magnetron sputtering system utilizes a small hollow cathode electron source which injects energetic electrons into magnetron plasma. The resultant plasma can be operated at significantly lowered pressure than conventional magnetron devices. If a small magnetron source is used, lift-off quality depositions have been attained with a target-to-workpiece distance of eight or more inches. The inherent limitation with such techniques, however, is the requirement that the magnetron source be quite small. For example, good quality lift-off depositions have been demonstrated with a magnetron three inches in diameter with a workpiece distance of nine inches. The limitation on deposition rate with the system, however, is about five to eight Angstroms per second due to the high-power density at the cathode. This limitation limits implementation of this technology on a manufacturing scale.
An additional problem, even with the above described hollow cathode enhanced magnetron, is due to the random trajectory of the deposited material as it leaves the magnetron surface and travels to the workpiece to be deposited which causes the impingement angles of the various particles as they strike a workpiece to be totally random. In other words, many of the particles strike the sample surface at acute angles. This allows them to form deposits on the sides or even underneath overhanging portions of the masks thus leading to the difficulties in lift-off mentioned above.
There is accordingly a distinct need for improvement in even the hollow cathode enhanced magnetron sputtering process in order to render it suitable for full-scale manufacturing operations.